Help me with boy friend issues?

So me and my bf have been dating for a month and my best friend went through his phone and saw some messages from another girl where they were calling each other baby and sending kisses. She also noticed that he sends her good morning and good night messages to us at the same time.. She also saw that he said he would kiss her before winter break.. So she asked him who that was and he replied with she is my friend, Best friend I guess. She then told me and I guess I'm just wondering if and how I should bring it up because they are good friends and I don't want to ruin their relationship... I really like him but I'm not even sure how I should go about this...